- Chilli peppers in the sandwiches might work - lots of really hot chilli peppers.
- Or you could get one of those fake blood capsules from a joke shop and put that in.
- Raw liver?
- If you can afford it, caviar (you can get cheaper varieties) usually leaves its mark around the lips and teeth, and it has a rather 'acquired' taste.
- Dog food, either in a sandwich or in a packet disguised as pate.
- Instead of milk, try making up some powdered baby formula, a little weaker than it should be, because it tastes foul enough anyway - weaker is fouler. Then leave it in a milk container.